Magnetic field sensors are in widespread commercial use with applications such as linear and rotary encoders, proximity detectors, and earth's field magnetometers. One common magnetic field sensor is based on the Hall effect and is used to sense magnetic fields in the range of 100 to 1000 Oersteads (Oe). Another common magnetic field sensor is based on the magnetoresistive (MR) effect in semiconductors or ferromagnetic materials, and is used to sense lower fields and fields at a greater distance than Hall-effect sensors. The MR magnetic field sensor detects magnetic field signals through the resistance changes of a sensing element, fabricated of a magnetic material, as a function of the magnitude and direction of magnetic flux being sensed by the sensing element.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the sensing element resistance varies as the square of the cosine of the angle between the magnetization in the sensing element and the direction of sense current flowing through the sensing element. The external magnetic field being sensed causes a change in the direction of magnetization in the sensing element, which in turn causes a change in resistance in the sensing element and a corresponding change in the sensed current or voltage.
Electrical bridge circuits made of AMR materials are used as magnetic field sensors to sense fields below approximately 50 Oe. An example of a magnetic field sensor using AMR elements in a Wheatstone bridge circuit is described in U.S. Pat. No. 5,247,278 assigned to Honeywell. Another example of an AMR Wheatstone bridge circuit, used in conjunction with a current strap to function as a current sensor, is described in IEEE Transactions on Magnetics, Vol. MAG-6, November 1976, pp. 813-815.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures. The essential feature of GMR is that there are at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as Fe/Cr, Co/Cu, or Co/Ru multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers, as well as in essentially uncoupled layered structures in which the magnetization orientation in one of the two ferromagnetic layers is fixed or pinned. The physical origin is the same in all types of GMR structures: the application of an external magnetic field causes a variation in the relative orientation of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes. A particularly useful application of GMR is a sandwich structure comprising two uncoupled ferromagnetic layers separated by a nonmagnetic metallic spacer layer in which the magnetization of one of the ferromagnetic layers is pinned. The pinning may be achieved by depositing the ferromagnetic layer to be pinned onto an iron-manganese (Fe-Mn) layer to exchange couple the two layers. This results in a spin valve (SV) sensor in which only the unpinned or free ferromagnetic layer is free to rotate in the presence of an external magnetic field. IBM's U.S. Pat. No. 5,206,590 discloses a basic SV sensor. IBM's U.S. Pat. No. 5,159,513 discloses an SV sensor in which at least one of the ferromagnetic layers is of cobalt or a cobalt alloy, and in which the magnetizations of the two ferromagnetic layers are maintained substantially perpendicular to each other at zero externally-applied magnetic field by exchange coupling of the pinned ferromagnetic layer to an antiferromagnetic layer. IBM's U.S. Pat. No. 5,341,261 describes an SV sensor having a thin film of cobalt adjacent to the metallic spacer layer for increased magnetoresistance. The SV sensor that has the most linear response and the widest dynamic range is one in which the magnetization of the pinned ferromagnetic layer is parallel to the signal field, and the magnetization of the free ferromagnetic layer is perpendicular to the signal field. The design and operation of an SV sensor is described by Heim et al. in "Design and Operation of Spin-Valve Sensors", IEEE Transactions on Magnetics, Vol. 30, No. 2, March 1994, pp. 316-321.
The use of GMR elements in a bridge circuit for a magnetic field sensor has been suggested by Daughton et al. in "GMR Materials for Low Field Applications", IEEE Transactions on Magnetics, Vol. 29, No. 6, November 1993, pp. 2705-2710. That reference suggests that a bridge circuit using a "pinned" GMR structure (i.e., an SV element) may be possible, but states that the device has not yet been demonstrated.
What is needed is a bridge circuit magnetic field sensor that takes advantage of the improved performance of an SV sensor over a conventional AMR sensor.